1. Field of the Invention
This invention relates to an improvement in the preparation of polysilanes according to a conventional Wurtz condensation method and more particularly, to a method of preparing high molecular weight polysilanes in high yields.
2. Prior Art
Polysilanes are regarded useful as precursors for silicon carbide ceramics, photoresist materials, photo-polymerization initiators, photoconductive materials, and non-linear optical materials. Since the discovery of soluble polysilanes (West, R. et al., J. Am. Chem. Soc., 1981, 103, 7352), researchers have strived for the development of a preparation method and application of polysilanes.
In the prior art, polysilanes are generally prepared by desalting condensation reaction of diorganodihalosilanes with alkali metals, which is known as Wurtz condensation method (see Kipping, F. S., J. Chem. Soc., 1921, 119, 830, and Burkhard, C., J. Am. Chem. Soc., 1949, 71, 963).
Although polysilanes can be prepared by the Wurtz condensation methods, they are obtained in low yields. Especially high molecular weight polysilanes which are regarded important from the standpoint of ease of handling in forming steps including film formation and spinning as well as formability are available in very low yields. By effecting reaction in aromatic solvents such as toluene or ether solvents, the yield of polysilanes can be increased at the sacrifice of molecular weight. For example, the proportion of high molecular weight polysilanes produced is only 20% in aromatic solvents and almost nil in ether solvents.
On the contrary, reaction in alkanes is known as effective for increasing the molecular weight while the percent yield can be increased by increasing the reaction temperature. Then the reaction in alkanes is advantageous when high molecular weight polysilanes are desired. For example, Trujillo (J. Organomet. Chem., 1980, 198, C27) and Taylor (GB 87-10531 or JP-A 286433/1988) reported that polymethylphenylsilane was obtained in yields of 65% and 57%, respectively. Their actual yields are 43% and 35% when a polysilane fraction having low dispersity is collected by a re-precipitation purification technique. Zeigler et al. disclose in WO 87-06234 or Japan Phase Publication 503237/1989 a process for preparing a polysilane in high yields by adjusting a relative solubility parameter with a solvent, but the yield is at most 45%.
The reason why the percent yield of high molecular weight polysilanes is low is that when diorganodihalosilanes are reacted with alkali metals, cyclic or chainlike oligosilanes and low molecular weight polysilanes are inevitably formed in addition to high molecular weight polysilanes, resulting in low yields of high molecular weight ones.
There is a demand for a method capable of preparing high molecular weight polysilanes in high yields.